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3.2 Bipolar Input to Channel A1
3.3 Analog Input – Channel B1
Analog Interface
This configuration allows single-ended signals of ± 2.5-V (+5-Vpp) to be applied to either the (+) or (-) input
of channel A1 (via J2 pin 1 or 3 referenced to pin 2). The input also can be applied to connector J4 (not
shown) pins 2 or 4, referred to analog ground.
By changing resistor components and setting the appropriate jumpers, it is possible to configure the input
buffer to accept bipolar input voltages. Table 1 is related to the schematic presented in Figure 1 and
represents just a few of the possible input configurations.
Table 1. Typical Analog Input Buffer Circuit Values
Input Voltage R3 R2 R1 R4 W2
R28 R25 R24 R29 W1
See Figure 1 Default open 5 k Ω 5 k Ω 5 k Ω 1–2
0 – +5
0 – 2.5 5 k Ω 5 k Ω open 5 k Ω 2–3
– 2.5 – +2.5 20 k Ω 4 k Ω 20 k Ω 4 k Ω 2–3
– 5 – +5 20 k Ω 4 k Ω 10 k Ω 2 k Ω 2–3
– 10 – +10 20 k Ω 4 k Ω 5 k Ω 1 k Ω 2–3
The output from the buffer stage in each case applies 0 V to 5 V to the CHA1(+) input when the applied
signal is connected to J2 pin 1 or J4 pin 2. The applied signal is directed to the CHA1(-) input when
connected via J2 pin 3 or J4 pin 4.
When operating the ADS7864 with single-ended signals it is important to keep the unused ADC input
biased to +2.5 V. This is easily accomplished on the EVM by changing the components associated with
either the inverting or noninverting only, leaving the default component values shown above on the
unused input. For example, to achieve a bipolar input range of ± 10 V on CHA1(+), use the component
values shown above for R1–R4 and move the shunt on W2 from position 1-2 to position 2-3. Components
R24, R25, R28, and R29 and the shunt on W1 should remain in the default conditions shown in Table 1
The analog input to the ADS7864MEVM board for channel B1 is comprised the INA159 difference
amplifier and the associated circuitry as shown in Figure 2 . The INA159 is powered from the +5-V analog
supply, and arranged as a noninverting amplifier with a gain of 0.2. The internal +2.5-V reference voltage
of the ADS7864 is applied to both REF1 and REF2 pins of the INA159 to provide a direct ± 10-V interface
with built-in level translation to the noninverting input of channel B1.
SLAU188 – September 2006 ADS7864MEVM 3
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